[wvh]

Metadata can be quite variable. Library, catalogue, picture tags. The majority of terms are common, but some can be pretty specific and (as a developer) you'd need to store them. You might not have control over the schema or even have a "finite" set of possibilities.

Imagine you want to store random metadata from a digital camera picture, or perhaps even XML/HTML attributes. You can create another table and add each new attribute – join on query – but if you don't plan to search for that data directly, it's easier to skip normalisation and dump the original set into a JSON(B) or HStore field. You don't have to add every possible attribute to your data model or schema, you can carry data along and not analyse it if it's not relevant to you.